Aluminium-copper-lithium alloy sheets for producing aeroplane fuselages

ABSTRACT

The invention concerns a sheet 0.5 to 8 mm thick made from aluminium alloy. The sheet can be obtained by a method comprising casting, homogenising, hot rolling and optionally cold rolling, solution heat treatment, quenching and tempering, the composition and the tempering being combined in such a way that the elasticity limit in the longitudinal direction R p0.2 (L) is between 395 and 435 MPa. A sheet according to the invention is particularly advantageous for producing aircraft fuselage panels.

SCOPE OF THE INVENTION

The invention relates to aluminum-copper-lithium alloy rolled products, and more particularly to such products, their manufacturing processes and use, intended notably for the field of aeronautical and aerospace construction.

STATE OF THE ART

Rolled products made of aluminum alloy are developed in order to produce fuselage components intended notably for the aeronautical and aerospace industry.

Aluminum-copper-lithium alloys are particularly beneficial for the production of this type of product.

U.S. Pat. No. 5,032,359 describes a vast family of aluminum-copper-lithium alloys in which the addition of magnesium and silver, in particular between 0.3 and 0.5 percent by weight, makes it possible to increase the mechanical strength.

U.S. Pat. No. 5,455,003 describes a process for manufacturing Al—Cu—Li alloys that have improved mechanical strength and fracture toughness at cryogenic temperature, in particular owing to appropriate strain hardening and aging. This patent particularly recommends the composition, expressed as a percentage by weight, Cu=3.0-4.5, Li=0.7-1.1, Ag=0-0.6, Mg=0.3-0.6 and Zn=0-0.75.

U.S. Pat No. 7,438,772 describes alloys including, expressed as a percentage by weight, Cu: 3-5, Mg: 0.5-2, Li:0.01-0.9 and discourages the use of higher lithium contents because of a reduction in the balance between fracture toughness and mechanical strength.

U.S. Pat. No. 7,229,509 describes an alloy including (% by weight): (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn, 0.4 max Zr or other grain-refining agents such as Cr, Ti, Hf, Sc, and V.

US patent application 2009/142222 A1 describes alloys including (% by weight), 3.4% to 4.2% Cu, 0.9% to 1.4% Li, 0.3% to 0.7% Ag, 0.1% to 0.6% Mg, 0.2% to 0.8% Zn, 0.1% to 0.6% Mn and 0.01% to 0.6% of at least one element for controlling the granular structure. This application also describes a process for manufacturing extruded products.

US patent application 2011/0247730 describes alloys including (% by weight), 2.75 to 5.0% Cu, 0.1 to 1.1% Li, 0.3 to 2.0% Ag, 0.2 to 0.8% Mg, 0.50 to 1.5% Zn, and up to 1.0% Mn, with a Cu/Mg ratio between 6.1 and 17, this alloy being insensitive to work hardening.

Patent application CN101967588 describes alloys of composition (% by weight) Cu 2.8-4.0; Li 0.8-1.9 ; Mn 0.2-0.6 ; Zn 0.20-0.80, Zr 0.04-0.20, Mg 0.20-0.80, Ag 0.1-0.7, Si<0.10. Fe<0.10, Ti<0.12.

US patent application 2011/209801 relates to wrought products such as extruded, rolled and/or forged aluminum alloy-based products comprising, as a percentage by weight, Cu: 3.0-3.9; Li: 0.8-1.3, Mg: 0.6-1.0; Zr: 0.05-0.18; Ag: 0.0-0.5; Mn: 0.0-0.5; Fe+Si<=0.20; at least one element from a group from among Ti: 0.01-0.15; Sc: 0.05-0.3; Cr: 0.05-0.3; Hf: 0.05-0.5; other elements <0.05 each and <0.15 in total, remainder aluminum, products being particularly useful in the production of thick aluminum products intended for producing structural elements in the aeronautical industry.

The required characteristics for aluminum sheets intended for fuselage applications are described, for example, in patent EP 1 891 247. It is notably desirable that the sheet has a high yield stress (to resist buckling) as well as high fracture toughness in plane strain, notably characterized by a high value of apparent stress intensity factor at break (K_(app)) and a long R-curve.

Patent EP 1 966 402 discloses an alloy comprising 2.1 to 2.8% by weight of Cu, 1.1 to 1.7% by weight of Li, 0.1 to 0.8% by weight of Ag, 0.2 to 0.6% by weight of Mg, 0.2 to 0.6% by weight of Mn, a quantity of Fe and Si less than or equal to 0.1% by weight each, and inevitable impurities with a content less than or equal to 0.05% by weight each and 0.15% by weight in total, the alloy being substantially free of zirconium, particularly suitable for obtaining recrystallized sheets.

Damage tolerance dimensioning consists in determining a limit size for detectable defects, for which it can be guaranteed that they will not lead to rupture during a defined time interval. To achieve this dimensioning it is necessary to know the behavior of cracks subjected to a representative load on panels of sufficient size. Furthermore in the case of the evaluation of large damage capability for which the undetected rupture of a stiffener is assumed, the width of the crack can be large and it is useful to have accurate fracture toughness data for very long cracks. The fracture toughness characterizations of sheets are generally carried out by the R-curve test on panels less than or equal to 760 mm wide. The R-curve test is a widely recognized method to characterize fracture toughness properties. The R-curve represents the evolution of the effective stress intensity factor for crack growth as a function of effective crack extension, under increasing monotonic loading. The R-curve enables one to determine the critical load for an unstable fracture for any configuration relevant to cracked aircraft structures. The values of the stress intensity factor and crack extension are actual values as defined in standard ASTM E561. It is generally considered that the width of the panel must not modify the level of the R-curve, namely the effective stress intensity factor for a given effective crack growth, but only the valid length of the curve. However, it has become apparent within the framework of this invention that this assumption is not always true, and that in fact the characterization on wider panels, such as 1220 mm wide panels, takes into account certain specific properties of the material not able to be deduced from characterizations performed on less wide panels. Thus, the state of the art knowledge is unable to predict which alloys and which thermomechanical treatments will allow the most advantageous properties to be attained for K_(app) and for the level of the R-curve of wide width panels, as properties will influence the damage tolerance dimensioning.

Furthermore, for certain fuselage applications, it is particularly important that the fracture toughness is high in the L-T direction. Indeed, in some configurations the bending stresses on the fuselage around the axis of the wings become critical, notably for the upper part of the fuselage. The cracks on the sheets, for which the longitudinal direction and also the longitudinal direction of the fuselage, are strained in the L-T direction.

There exists a need for metal sheets measuring 0.5 to 8 mm thick, made of aluminum-copper-lithium alloy presenting improved properties as compared with those of known products, particularly in terms of fracture toughness measured on wide width panels notably in the L-T direction, static mechanical strength and corrosion resistance properties, while having low density.

OBJECT OF THE INVENTION

The object of the invention is an aluminum based alloy sheet of thickness 0.5 to 8 mm comprising

-   -   2.6 to 3.0% by weight of Cu,     -   0.5 to 0.8% by weight of Li,     -   0.1 to 0.4% by weight of Ag,     -   0.2 to 0.7% by weight of Mg,     -   0.06 to 0.20% by weight of Zr,     -   0.01 to 0.15% by weight of Ti,     -   optionally at least one element chosen among Mn, V, Cr, Sc, and         Hf, the quantity of element, if chosen, being from 0.01 to 0.8%         by weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to 0.3% by         weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5% by         weight for Hf,     -   a quantity of Zn less than 0.2% by weight, a quantity of Fe and         Si less than or equal to 0.1% by weight each, and inevitable         impurities having a content less than or equal to 0.05% by         weight each and 0.15% by weight in total,         said sheet being obtained by a method comprising casting,         homogenization, hot rolling and optionally cold rolling,         solution heat treatment, quenching and aging, the composition         and the aging being combined in such a way that the yield stress         in the longitudinal direction R_(p0.2)(L) is between 395 and 435         MPa.

Another object of the invention is the method of manufacturing a sheet according to the invention of 0.5 to 8 mm in thickness made of an aluminum-based alloy wherein, successively

-   a) a molten metal bath is prepared comprising     -   2.6 to 3.0% by weight of Cu,     -   0.5 to 0.8% by weight of Li,     -   0.1 to 0.4% by weight of Ag,     -   0.2 to 0.7% by weight of Mg,     -   0.06 to 0.20% by weight of Zr,     -   0.01 to 0.15% by weight of Ti,     -   optionally at least one element chosen among Mn, V, Cr, Sc, and         Hf, the quantity of element, if chosen, being from 0.01 to 0.8%         by weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to 0.3% by         weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5% by         weight for Hf,     -   a quantity of Zn less than 0.2% by weight, a quantity of Fe and         Si less than or equal to 0.1% by weight each, and inevitable         impurities having a content less than or equal to 0.05% by         weight each and 0.15% by weight in total, -   b) a slab is cast from said molten metal bath; -   c) said slab is homogenized at a temperature between 450° C. and     535° C.; -   d) said slab is hot rolled and optionally cold rolled into a sheet     of thickness between 0.5 mm and 8 mm; -   e) said sheet is solution heat treated at a temperature of between     450° C. and 535° C. and quenched; -   h) said sheet undergoes controlled stretching with a permanent     deformation of 0.5 to 5%, total cold working after solution heat     treatment and quenching being less than 15%; -   i) aging is performed comprising heating to a temperature between     130° C. and 170° C. and preferably between 150° C. and 160° C. for 5     to 100 hours and preferably from 10 to 40 hours, the composition and     aging being combined so that the yield stress in the longitudinal     direction R_(p0.2)(L) is between 395 and 435 MPa.

Yet another object of the invention is the use of a sheet according to the invention in an aircraft fuselage panel.

DESCRIPTION OF THE FIGURES

FIG. 1—R-curves obtained in the L-T direction on sheets of thickness 4 to 5 mm for test pieces measuring 760 mm and 1220 mm wide.

FIG. 2—R-curves obtained in the L-T direction on sheets of thickness 1.5 to 2 5 mm for test pieces measuring 760 mm and 1220 mm wide.

FIG. 3—R-curves obtained in the L-T direction on E#1 sheets having undergone various tempering for test pieces measuring 760 mm and 1220 mm wide.

FIG. 4—R-curves obtained in the L-T direction on E#2 sheets having undergone various aging for test pieces measuring 760 mm and 1220 mm wide.

FIG. 5—Relationship between the yield stress in the longitudinal direction and the stress intensity factor K_(app) L-T measured on test samples measuring 1220 mm wide for sheets 4 to 5 mm thick.

FIG. 6—Relationship between the yield stress in the longitudinal direction and the stress intensity factor K_(app) L-T measured on test samples measuring 1220 mm wide for sheets 1.5 to 2.5 mm thick.

DESCRIPTION OF THE INVENTION

Unless otherwise specified, all the indications concerning the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content, expressed as a percentage by weight is multiplied by 1.4. The designation of alloys is compliant with the rules of The Aluminum Association, known to experts in the field. The density depends on the composition and is determined by calculation rather than by a method of weight measurement. The values are calculated in compliance with the procedures of The Aluminum Association, which is described on pages 2-12 and 2-13 of “Aluminum Standards and Data”. Unless otherwise specified, the definitions of metallurgical states listed in European Standard EN 515 apply. The static mechanical properties under stretching, in other words the ultimate tensile strength R_(m), the conventional yield strength at 0.2% offset (R_(p0.2)) and elongation at break A %, are determined by a tensile test according to standard EN ISO 6892-1, and sampling and test direction being defined by standard EN 485-1. Within the framework of the invention, the mechanical properties are measured in full thickness.

Within the framework of the present invention, a essentially unrecrystallized granular structure refers to a granular structure such that the recrystallization rate at ½ thickness is less than 30% and preferably less than 10% and a essentially recrystallized granular structure refers to a granular structure such that the recrystallization rate at ½ thickness is greater than 70% and preferably greater than 90%. The recrystallization rate is defined as the area fraction on a metallographic section occupied by recrystallized grains.

A curve giving the effective stress intensity factor as a function of the effective crack extension, known as the R-curve, is determined according to standard ASTM E 561. The critical stress intensity factor K_(C), in other words the intensity factor which makes the crack unstable, is calculated from the R-curve. The stress intensity factor K_(CO) is also calculated by allotting the initial crack length at the beginning of the monotonic load, at critical load. These two values are calculated for a test piece of the required shape. K_(app) represents the K_(CO) factor corresponding to the test piece that was used to perform the R-curve test. K_(eff) represents the K_(C) factor corresponding to the test piece that was used to perform the R-curve test. Δa_(eff) (max) represents the crack extension of the last point of the R-curve, valid according to standard ASTM E561. The last point is obtained either at the time of sudden rupture of the test piece, or possibly when the stress on the uncracked ligament exceeds the yield stress of the material. Unless otherwise specified, the crack size at the end of the stage of pre-cracking by fatigue is W/3 for test pieces of the M(T) type, wherein W is the width of the test piece as defined in standard ASTM E561.

Unless otherwise specified, the definitions of standard EN 12258 apply.

Sheets measuring 0.5 to 8 mm thick made of Al—Cu—Li alloy according to the composition of the invention allow, when their yield stress in the longitudinal direction R_(p0.2)(L) is between 395 and 435 MPa, a particularly advantageous fracture toughness to be obtained on wide width panels in the L-T direction.

The present inventors noted that, surprisingly, the fracture toughness in the L-T direction on 1220 mm wide panels is improved for a precise range of yield stress values in the longitudinal direction R_(p0.2)(L) whereas this effect is not observed when the measurement is carried out on 760 mm wide panels. Thus, within the framework of the invention, it was observed that there is an optimal yield stress value range that is specific to the width 1220 mm, which cannot be interpreted by considerations based on the plasticization of the uncracked ligament, underlying the limits of validity of standard ASTM E561. The present inventors have thus established that sheets obtained by a method comprising casting, homogenization, hot rolling and optionally cold rolling, solution heat treatment, quenching and aging possess advantageous properties when the composition and aging are combined in such a way that the yield stress in the longitudinal direction R_(p0.2)(L) is between 395 and 435 MPa.

For certain compositions according to the invention, the sheets have advantageous properties when the aging is performed to “peak”. Within the framework of the present invention and for the sake of simplicity, aging to “peak” refers to an aging process for which the yield stress in the transverse direction R_(p0.2)(TL) has a value of at least 95% of the yield stress in the transverse direction R_(p0.2)(TL) obtained for an aging process having an equivalent time at 155° C. of 48 hours. Within the framework of the present invention, aging performed to “peak” is preferred. For other compositions according to the invention, under-aging may be necessary to reach the desired yield stress. However, if under-aging is excessive, certain properties of the sheets, notably the thermal stability, are not satisfactory. Within the framework of the present invention, thermal stability refers to the stability of the mechanical properties during an exposure to temperatures representative of conditions experienced in civil aviation, being simulated by aging of 1000 hours at 85° C. for example. Therefore, if necessary, under-aging is performed for which the yield stress in the transversal direction R_(p0.2)(TL) has a value between 88% and 94% and preferably at least 91% of the value obtained for aging having an equivalent time of 48 hours at 155° C. The copper content of the products according to the invention lies between 2.6 and 3.0% by weight. In an advantageous embodiment of the invention, the copper content lies between 2.8 and 3.0% by weight. In an advantageous embodiment of the invention, the copper content is at most 2.95% by weight and advantageously at most 2.9% by weight. When the copper content is too high, the yield stress R_(p0.2)(L) is too high to be advantageous in the under-aging conditions according to the invention. When the copper content is too low, the minimum static mechanical properties are not achieved, even for aging to peak.

The lithium content of the products according to the invention lies between 0.5 and 0.8% by weight. Advantageously, the lithium content lies between 0.55 and 0.75% by weight. Preferably, the lithium content lies between 0.60% and 0.73% by weight The addition of lithium can contribute to increased mechanical strength and fracture toughness. Lithium content that is too high or too low does not allow a high value of fracture toughness and/or a sufficient yield stress to be obtained.

The magnesium content of the products according to the invention lies between 0.2 and 0.7% by weight, preferably between 0.25 and 0.50% by weight and most preferably between 0.30 and 0.45% by weight. In an advantageous embodiment of the invention, the magnesium content is at most 0.4% by weight.

The zirconium content lies between 0.06 and 0.20% by weight and preferably between 0.10 and 0.18% by weight. When a essentially unrecrystallized granular structure is preferred, the zirconium content is advantageously between 0.14 and 0.17% by weight.

The silver content lies between 0.1 and 0.4% by weight. In an advantageous embodiment of the invention, the silver content lies between 0.2 and 0.3% by weight. In an embodiment of the invention, the silver content lies between 0.15 and 0.28% by weight.

The titanium content lies between 0.01 and 0.15% by weight. The addition of titanium helps to control the granular structure, particularly during casting.

The alloy can optionally contain at least one element selected from Mn, V, Cr, Sc, and Hf, the quantity of the element, if chosen, being from 0.01 to 0.8% by weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to 0.3% by weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5% by weight for Hf. These elements can contribute to controlling the granular structure. In an embodiment of the invention, Mn, V, Cr or Sc is not added and their content is less than or equal to 0.05% by weight.

Preferably, the iron and silicon contents are each at the most 0.1% by weight. In an advantageous embodiment of the invention, the iron and silicon contents are at most 0.08% by weight and preferably at most 0.04% by weight. A controlled and limited iron and silicon content helps to improve the balance between mechanical strength and damage tolerance.

The zinc content is less than 0.2% by weight and preferably less than 0.1% by weight. The zinc content is advantageously less than 0.04% by weight.

The inevitable impurities are kept at a content less than or equal to 0.05% by weight each and 0.15% by weight in total.

The sheet manufacturing method according to the invention comprises steps of preparing, casting, rolling, solution heat treatment, quenching, controlled stretching and ag ing. In a first step, a molten metal bath is prepared to obtain an aluminum alloy of composition according to the invention.

The molten metal bath is then cast in the form of a rolling slab.

The rolling slab is then homogenized at a temperature between 450° C. and 535° C. and preferably between 480° C. and 530° C. The homogenization time is preferably between 5 and 60 hours.

After homogenization, the rolling slab is generally cooled at room temperature before being preheated ready for hot working. The aim of preheating is to reach a temperature preferably between 400° C. and 500° C. enabling the deformation by hot rolling to take place.

Hot rolling, and optionally cold rolling, is carried out to obtain to a thickness of 0.5 to 8 mm Intermediate heat treatments during and/or after the rolling may be carried out in some cases. Preferably, however, the method does not include intermediate heat treatment during and/or after the rolling. The sheet thus obtained is then solution heat treated by thermal treatment between 450° C. and 535° C., preferably for 5 min to 8 hours, then quenched. It is known to those skilled in the art that the precise solution heat treatment conditions must be chosen based on the thickness and the composition so as to place the hardening elements in a solid solution.

The sheet then undergoes cold working by controlled stretching with a permanent deformation of 0.5 to 5% and preferably of 1 to 3%. Known steps such as rolling, flattening, straightening or shaping may optionally be performed after heat treatment and quenching and before or after controlled stretching.

However, the total cold working after solution heat treatment and quenching must remain below 15% and preferably less than 10%. Significant cold working after solution heat treatment and quenching result in the appearance of numerous shearing bands through several grains; these shearing bands not being desirable.

Aging is carried out at a temperature between 130° C. and 170° C. and preferably between 150° C. and 160° C. for 5 to 100 hours and preferably from 10 to 40 hours so as to reach a yield stress in the longitudinal direction R_(p0.2)(L) between 395 and 435 MPa. In an embodiment of the invention wherein the granular structure is essentially recrystallized, a yield stress in the longitudinal direction R_(p0.2)(L) between 395 and 415 MPa may be preferred in some cases. In another embodiment of the invention wherein the granular structure is essentially unrecrystallized, a yield stress in the longitudinal direction R_(p0.2)(L) between 415 and 435 MPa may be preferred in some cases.

Advantageously, the composition reaches the desired longitudinal elasticity limit with an equivalent time at 155° C. less than 48 h and preferably less than 30 h. Preferably, the final temper is T8.

The equivalent time t_(i), at 155° C. is defined by the formula:

$t_{i}\frac{\int{{\exp \left( {{- 16400}/T} \right)}{t}}}{\exp \left( {{- 16400}/T_{ref}} \right)}$

where T (in Kelvin) is the instantaneous treatment temperature of the metal, which changes with time t (in hours), and T_(ref) is a reference temperature fixed at 428 K. t_(i) is expressed in hours. The constant Q/R=16400 K is derived from the activation energy of the diffusion of Cu, for which the value Q=136100 J/mol was used. The present inventors noted in particular that the preferred range of magnesium content helps limit the aging time leading to a favorable compromise of properties.

In an embodiment of the invention, a short thermal treatment is carried out after controlled stretching and before aging so as to improve the formability of the sheets. The slabs can thus be formed by a process such as draw-forming before being aged.

The most favorable granular structure depends on the thickness of the products.

The sheets according to the invention with thickness between 0.5 and 3.3 mm, advantageously have the following properties

-   -   a fracture toughness in plane strain Kapp, measured on test         pieces of type CCT760 (2ao=253 mm), in the L-T direction of at         least 120 MPa√m and     -   a fracture toughness in plane strain Kapp, measured on test         pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at         least 120 MPa√m.

The present inventors have further noted that for the sheets according to the invention with thickness between 0.5 and 3.3 mm and preferably between 1.0 and 3.0 mm, the fracture toughness in plane strain Kapp in the L-T direction is higher for sheets for which the structure is essentially recrystallized. Thus, sheets with thickness between 0.5 and 3 3 mm and preferably between 1.0 and 3 0 mm and whose granular structure is essentially recrystallized, advantageously have the following properties:

-   -   a fracture toughness in plane strain Kapp, measured on test         pieces of type CCT760 (2ao=253 mm), in the L-T direction of at         least 140 MPa√m and     -   a fracture toughness in plane strain Kapp, measured on test         pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at         least 150 MPa√m. The sheets according to the invention with         thickness between 3.4 and 6 mm, and advantageously have the         following properties     -   a fracture toughness in plane strain Kapp, measured on test         pieces of type CCT760 (2ao=253 mm), in the L-T direction of at         least 150 MPa√m and preferably at least 155 MPa√m and     -   a fracture toughness in plane strain Kapp, measured on test         pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at         least 170 MP√am and preferably at least 180 MPa√m.

Advantageously, the granular structure of sheets with thickness between 3.4 and 8 mm and preferably between 4 and 8 mm is essentially unrecrystallized. The intercrystalline corrosion resistance of the sheets according to the invention is high. In a preferred embodiment of the invention, the sheet of the invention can be used without cladding.

The use of sheets according to the invention in an aircraft fuselage panel is advantageous. The sheets according to the invention are also advantageous in aerospace applications such as the manufacture of rockets.

EXAMPLES Example 1

In this example, Al—Cu—Li alloy sheets were prepared.

Five slabs, the composition of which is given in Table 1, were cast. Compositions B, C, D and E are according to the invention.

TABLE 1 Composition as a percentage by weight Reference Cu Li Mg Zr Ag Fe Si Ti A 3.2 0.73 0.68 0.14 0.26 0.03 0.04 0.03 B 3.0 0.70 0.64 0.17 0.27 0.02 0.03 0.03 C 3.0 0.73 0.35 0.15 0.27 0.02 0.03 0.03 D 2.7 0.75 0.58 0.14 0.28 0.03 0.02 0.03 E 2.9 0.73 0.45 0.14 0.29 0.04 0.02 0.03

The slabs were homogenized 12 hours at 505° C. The slabs were hot rolled to obtain sheets with a thickness of between 4.2 to 6.3 mm Certain sheets where then cold rolled to a thickness between 1.5 and 2.5 mm. The detail of sheets obtained and the aging conditions is given in Table 2.

TABLE 2 detail of sheets obtained and aging conditions Thickness Thickness Aging after hot after cold time at Reference rolling (mm) rolling (mm) 155° C. (h) A#1 4.2 — 36 A#2 4.4 1.5 36 B#1 4.6 — 36 B#2 4.4 1.5 36 C#1 4.3 — 24 C#2 4.4 1.5 24 D#1 4.3 — 40 D#2 6.3 2.5 40 E#1 4.3 — 36 E#2 6.3 2.5 36

After hot rolling and possibly cold rolling, the sheets were solution heat treated at 505° C. then smoothed out, stretched with a permanent elongation of 2% and aging. The aging conditions are not all identical since the increase in the yield stress with the aging time differs from one alloy to another. An attempt was made to obtain a yield stress at “peak” while limiting aging time. The aging conditions are given in Table 2.

The granular structure of the test samples was characterized based on microscopic observation of cross sections after anodic oxidation under polarized light.

The granular structure of the sheets was essentially unrecrystallized for all the sheets except for sheets D#2 and E#2 for which the granular structure was essentially recrystallized.

The test samples were mechanically tested to determine their static mechanical properties as well as their resistance to fatigue crack propagation. The yield stress under tension, the ultimate strength and elongation at rupture are given in Table 3.

TABLE 3 Mechanical properties expressed in MPa (R_(p0.2), R_(m)) or in percentage (A %) R_(p0.2)(TL)/ R_(p0.2) R_(p0.2)(TL) (TL) 48 h _(Rp0.2) Rm A % R_(p0.2) R_(m) A % 48 h 155° C. Reference (L) (L) (L) (TL) (TL) (TL) 155° C. (%) A#1 469 513 12.2 439 481 15.8 457 96% A#2 475 522 11.7 441 489 14.0 B#1 431 483 13.5 419 462 16.1 425 99% B#2 431 486 12.9 414 460 17.1 C#1 430 471 13.6 411 455 15.5 434 95% C#2 423 472 12.2 399 451 15.9 D#1 420 462 13.0 384 428 16.3 407 94% D#2 403 437 11.6 371 428 13.9 E#1 453 487 12.5 428 464 15.9 433 99% E#2 433 464 11.4 395 458 11.4

Table 4 summarizes the fracture toughness test results on CCT test pieces of width 760 mm for these test samples.

Table 4 summarizes the results of R-curves for test pieces of width 760 mm

Kapp Kr60 Δaeff max [MPa√m] [MPa√m] [mm] Reference T-L L-T T-L L-T T-L L-T A#1 187 161 247 213 166 80 A#2 160 114 210 151 185 103 B#1 180 178 238 238 171 180 B#2 167 124 223 166 152 144 C#1 182 165 242 219 134 151 C#2 154 127 203 162 165 110 D#1 174 150 230 200 238 163 D#2 147 151 196 201 222 210 E#1 181 159 240 213 241 132 E#2 137 164 181 219 161 214

Table 5 summarizes the fracture toughness test results for the R-curves obtained with CCT test pieces of width 1220 mm in the L-T direction.

Table 5 results of R-curves for test pieces of width 1220 mm in the L-T direction.

Δa_(eff) Kapp Kr60 max Reference [MPa√m] [MPa√m] [mm] A#1 169 202 172 A#2 117 138 247 B#1 176 209 281 B#2 120 145 193 C#1 191 224 237 C#2 120 134 106 D#1 175 206 244 D#2 180 213 244 E#1 159 192 139 E#2 167 196 187

The R-curves obtained for the sheets with thickness in the order of 4 mm are shown in FIG. 1. The R-curves obtained for the sheets with thickness of 1.5 to 2 5 mm are shown in FIG. 2. The points obtained after the last valid point according to standard ASTM E561 were represented.

Surprisingly, it is found that K_(app) L-T is substantially identical for 760 mm wide test pieces and 1220 mm wide test pieces for some sheets, while for other sheets K_(app) L-T is lower for 760 mm wide test pieces and for 1220 mm wide test pieces.

Example 2

In this example, the effect of the aging conditions was studied on the fracture toughness of Al—Cu—Li alloy sheets of composition according to the invention.

Following treatment identical to that of example 1, except for aging, sheets made of alloy E underwent aging for 20 h at 155° C. or for 25 h at 155° C.

The granular structure is not changed by these aging conditions. The test samples were mechanically tested to determine their static mechanical properties as well as their resistance to fatigue crack propagation. The yield stress under tension, the ultimate strength and elongation at rupture are given in Table 6.

TABLE 6 Mechanical properties expressed in MPa (R_(p0.2), R_(m)) or in percentage (A %) R_(p0.2)(TL)/ R_(p0.2)(TL) Aging 48 h duration R_(p0.2) R_(m) A % R_(p0.2) R_(m) A % 155° C. Reference A155° C. (L) (L) (L) (TL) (TL) (TL) (%) E#1 20 h 422 470 13 390 440 16.5 90% E#2 20 h 411 450 12.4 374 443 12 E#1 25 h 442 483 12.4 415 456 15.7 96% E#2 25 h 431 466 11.1 391 455 11.7 E#1 36 h 453 487 12.5 428 464 15.9 99% E#2 36 h 433 464 11.4 395 458 11.4

The R-curves characterized for a test piece of width 760 mm and 1220 mm in the L-T direction are given in FIGS. 3 (thickness 4.3 mm) and 4 (thickness 2 5 mm) and in Table 7. The points obtained after the last valid point according to standard ASTM E561 were represented.

TABLE 7 Results of R-curves for test pieces of width 760 mm and 1220 mm in the L-T direction. Test piece 760 mm Test piece 1220 mm Aging Δaeff Δaeff time at Kapp Kr60 max Kapp Kr60 max Reference 155° C. [MPa√m] [MPa√m] [mm] [MPa√m] [MPa√m] [mm] E#1 20 h 168 219 173 180 216 208 E#2 20 h 163 216 235 183 216 201 E#1 25 h 160 211 146 170 198 192 E#2 25 h 161 214 193 175 212 205 E#1 36 h 159 213 102 158 190 172 E#2 36 h 164 219 214 167 196 187

FIGS. 5 and 6 summarize all the results obtained. 

1. A sheet measuring 0.5 to 8 mm thick made of an aluminum-based alloy composition comprising 2.6 to 3.0% by weight of Cu, 0.5 to 0.8% by weight of Li, 0.1 to 0.4% by weight of Ag, 0.2 to 0.7% by weight of Mg, 0.06 to 0.20% by weight of Zr, 0.01 to 0.15% by weight of Ti, optionally at least one element chosen among Mn, V, Cr, Sc, and Hf, the quantity of element, if chosen, being from 0.01 to 0.8% by weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to 0.3% by weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5% by weight for Hf, a quantity of Zn less than 0.2% by weight, a quantity of Fe and Si less than or equal to 0.1% by weight each, and inevitable impurities having a content less than or equal to 0.05% by weight each and 0.15% by weight in total, said sheet being obtained by a method comprising casting, homogenization, hot rolling and optionally cold rolling, solution heat treatment, quenching and aging, the composition and the aging being combined in such a way that the yield stress in the longitudinal direction R_(p0.2)(L) is between 395 and 435 Mpa.
 2. The sheet according to claim 1, the copper content of which lies between 2.8 and 3.0% by weight and optionally between 2.8 and 2.9% by weight.
 3. The sheet according to claim 1, the lithium content of which lies between 0.55 and 0.75% by weight and optionally between 0.60 and 0.73% by weight.
 4. The sheet according to claim 1, the silver content of which lies between 0.2 and 0.3% by weight.
 5. The sheet according to claim 1, the magnesium content of which lies between 0.25 and 0.50% by weight and optionally between 0.30 and 0.45% by weight.
 6. The sheet according to claim 1 for which the aging is performed at “peak”.
 7. The sheet according to claim 1, with thickness between 0.5 and 3 3 mm and having the following properties a fracture toughness in plane strain K_(app), measured on test pieces of type CCT760 (2ao=253 mm), in the L-T direction of at least 120 MPa√m and a fracture toughness in plane strain K_(app), measured on test pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at least 120 MPa√m.
 8. The sheet according to claim 7, the granular structure of which is essentially recrystallized and having the following properties a fracture toughness in plane strain Kapp, measured on test pieces of type CCT760 (2ao=253 mm), in the L-T direction of at least 140 MPa√m and a fracture toughness in plane strain Kapp, measured on test pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at least 150 MPa√m.
 9. The sheet according to claim 1, with thickness between 3.4 and 6 mm and having the following properties a fracture toughness in plane strain K_(app), measured on test pieces of type CCT760 (2ao=253 mm), in the L-T direction of at least 150 MPa√m and optionally at least 155 MPa√m and a fracture toughness in plane strain K_(app), measured on test pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at least 170 MPa√m and optionally at least 180 MPa√m.
 10. The sheet according to claim 1, with thickness between 3.4 and 8 mm and optionally between 4 and 8 mm and the granular structure of which is essentially unrecrystallized.
 11. A method of manufacturing a sheet according to claim 1 of thickness of 0.5 to 8 mm made of an aluminum based alloy composition, said method comprising, successively a) a molten metal bath is prepared comprising 2.6 to 3.0% by weight of Cu, 0.5 to 0.8% by weight of Li, 0.1 to 0.4% by weight of Ag, 0.2 to 0.7% by weight of Mg, 0.06 to 0.20% by weight of Zr, 0.01 to 0.15% by weight of Ti, optionally at least one element chosen among Mn, V, Cr, Sc, and Hf, the quantity of element, if chosen, being from 0.01 to 0.8% by weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to 0.3% by weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5% by weight for Hf, a quantity of Zn less than 0.2% by weight, a quantity of Fe and Si less than or equal to 0.1% by weight each, and inevitable impurities having a content less than or equal to 0.05% by weight each and 0.15% by weight in total, b) a slab is cast from said molten metal bath; c) said slab is homogenized at a temperature between 450° C. and 535° C.; d) said slab is hot rolled and optionally cold rolled into a sheet of thickness between 0 5 mm and 8 mm; e) said sheet is solution heat treated at a temperature of between 450° C. and 535° C. and quenched; h) said sheet undergoes controlled stretching with a permanent deformation of 0.5 to 5%, total cold working after solution heat treatment and quenching being less than 15%; i) aging is performed comprising heating to a temperature between 130° C. and 170° C. and optionally between 150° C. and 160° C. for 5 to 100 hours and optionally from 10 to 40 hours, the composition and aging being combined so that the yield stress in the longitudinal direction R_(p0.2)(L) is between 395 and 435 MPa.
 12. A sheet according to claim 1 shaped into an aircraft fuselage panel. 